In digital communications intended for information transmission, various modulation methods have been utilized for transmitting large quantities of information at higher speeds while using the same power. The minimum unit used for transmitting this information is a symbol, and a communication frame is composed of a plurality of such symbols arranged in chronological order. The greater the amount of information transmittable by a single symbol, the greater the amount of information that can be transmitted over a predetermined time throughout the entire communications system. For instance, 4-bit information is provided by 1 symbol in a 16-level QAM modulation type signal (hereinafter referred to as a “16-QAM communication signal”). In addition, in a QAM modulation system, depending on the amplitude and phase settings that can be used, there also exist multilevel QAM communication signals other than the 16-level QAM communication signal.
In such a multilevel QAM communication signal, in order to detect symbol timing in a communication frame, a unique word UW is inserted at the beginning and, furthermore, a pilot symbol PS is inserted every predetermined number of symbols. For example, in the 16-QAM communication signal used by Inmarsat, a 40-symbol unique word UW is inserted at the beginning and a single-symbol pilot symbol PS is inserted every 29 symbols. These unique words UW and pilot symbols PS are preset signals and, in order to facilitate detection of symbol timing, they are typically set up as signals that look like BPSK signals.
However, in methods that involve detection of unique words UW and pilot symbols PS in this manner, the accuracy of symbol timing detection decreases when the unique word UW is short and a large number of samples is required for increasing the symbol timing detection timing to a predetermined accuracy when Eb/No is low. For this reason, symbol timing either cannot be detected or, even if it is detected, it may not be possible to detect it frequently.
As a method of overcoming this problem, Patent Document 1 and Patent Document 2 disclosed symbol synchronization devices, in which a reference signal corresponding to a symbol timing signal is produced in a receiver and this signal is compared with a received multilevel QAM communication signal to detect symbol timing. In addition, Patent Document 3 disclosed a detection method, in which a predetermined threshold value is set up in advance, the amplitude of a received signal is observed chronologically while comparing it with the above-mentioned threshold value, and the times, at which a chronological characteristic curve of the amplitude crosses a line representative of the threshold value, is used as the symbol timing.
[Patent Document 1] JP 2001-24562A
[Patent Document 2] JP 2001-24563A
[Patent Document 3] JP 2003-309613A